If they took only 20 mins to demolish each lockon then your concrete doesn’t sound up to scratch. Don’t mean to brag but a similar lock on I once built took them 6 hours to demolish with jack hammers and angle grinders. In future you need to include some metal reinforcing mesh or crinkly rebar rods, either offcuts of the proper stuff from builders skips or sawn up supermarket trolleys are ok. Including metal mesh serves not only to bind the concrete together but it spreads the shockwave from jack hammers throughout the material rather than allowing it to be concentrated at the chisel tip. Also good to include some short lengths of blue 6mm rope which prevents the forces of darkness from being able to separate the chunks of fragmented concrete which really winds them up. The other really important things are to use freshly bought cement, be sure your concrete mix is very thoroughly mixed – absolutely no sandy or cementy streaks visible – and then poured in less than 20 mins or so and when pouring it, don’t do it all in one go but ram or tamp (jiggle and shake) it in thoroughly into the container in stages with the end of a bit of 4 x 2. Your concrete looks like it has voids in it which is always a considerable weakness. Good luck with future courageous actions.
Never ever use ordinary “building” or soft sand which is only used for bricklaying and will make very crumbly concrete indeed. You should be using all in ballast which is sharp sand already mixed with pebbles. In some parts of the country – mostly the north – you can’t get this so will have to mix your own using SHARP sand and mixed size pebbles from pea size up to the size of brazil nuts or so.